Merge branch 'jc/rebase-orig-head'

* jc/rebase-orig-head:
  Documentation: mention ORIG_HEAD in am, merge, and rebase
  Teach "am" and "rebase" to mark the original position with ORIG_HEAD

git-am.sh

@@ -242,6 +242,7 @@ else
 		: >"$dotest/rebasing"
 	else
 		: >"$dotest/applying"
+		git update-ref ORIG_HEAD HEAD
 	fi
 fi